1. Field of the Invention
The present invention relates to pressure-type flow rate controllers for a variety of fluids including gases, for use in the manufacture of semiconductors, chemicals, precision machine parts, and the like. More specifically, this invention relates to a method and apparatus for the detection of the clogging of the orifice in the flow rate controller.
2. Description of the Prior Art
In the past, mass flow rate controllers have been widely used for control with precision of the flow in fluid feeding systems for facilities for manufacturing semiconductors and chemicals.
However, the mass flow rate controller has presented various problems including: (1) relatively slow response in the case of the heat type flow rate sensor, (2) poor control precision in low flow rate region and lack of product-to-product precision uniformity, (3) frequent occurrence of operating troubles and low operation reliability, and (4) high price and expensive replacement parts, which means high running costs.
Seeking a solution to those problems, the inventors of the present invention conducted extensive research and succeeded in developing a pressure-type flow rate control system using an orifice as disclosed in Japanese patent application laid open unexamined under 08-338546.
The pressure-type flow rate control system works on this principle: It has been known that the flow velocity of a gas passing through a nozzle reaches the sonic velocity if the ratio of the gas pressure upstream of the nozzle to the pressure downstream of the same--P.sub.2 /P.sub.1 where P.sub.1 =pressure on the upstream side and P.sub.2 =pressure on the downstream side--falls below the critical pressure of the gas (in the case of air, nitrogen, etc., about 0.5). In such a state, a change in pressure on the downstream side of the nozzle is no longer propagated to the upstream side. The flow rate of the gas passing through the orifice will be proportional with the pressure P.sub.1 on the upstream side of the orifice. As a result, it will be possible to obtain a stabilized mass flow rate corresponding to the state on the upstream side of the nozzle.
In other words, in the event that the orifice diameter is fixed, if the upstream pressure P.sub.1 is held twice or more higher than the downstream pressure P.sub.2, then the downstream flow rate Q.sub.C of the gas passing through the orifice will be dependent on the upstream pressure P.sub.1 only, with a linearity given by the equation Q.sub.C =KP.sub.1 (K=constant) being established to a high degree of precision. And, if the orifice is the same, so is the constant K.
The construction of this pressure-type flow rate control system will now be described with reference to FIG. 12.
The flow passage 4 upstream of the orifice 2 is connected to a control valve CV that is operated by a drive 8. The downstream flow passage 6 is led to a fluid reactor (not shown) via a gas take-out joint 12.
The pressure P.sub.1 on the upstream side of the orifice 2 is detected by a pressure detector 14, and then sent to an amplifier circuit 16 and displayed on a pressure indicator 22. The output is then digitized by an A/D converter 18 and referred to a calculation circuit 20 where the flow rate Q on the downstream side of the orifice 2 is worked out with the equation Q=KP.sub.1 (K=constant).
Meanwhile, the upstream temperature T.sub.1 is detected by a temperature detector 24 and output to a temperature revision circuit 30 via an amplifier circuit 26 and an A/D converter 28. There, the flow rate Q is revised for the temperature, and the calculated flow rate Q.sub.C is output to a comparison circuit 36. The calculation circuit 20, the temperature revision circuit 30, and the comparison circuit 36 form a calculation control circuit 38.
A flow rate setting circuit 32 outputs a set flow Q.sub.S to the comparison circuit 36 through an A/D converter 34. The comparison circuit 36 works out a signal difference Q.sub.Y between the calculated flow rate Q.sub.C and the set flow Q.sub.S with the equation Q.sub.Y =Q.sub.C -Q.sub.S and outputs that signal difference Q.sub.Y to the drive 8 through an amplifier circuit 40. The drive 8 regulates the control valve CV in such a way as to bring the difference signal Q.sub.Y to zero, that is, to equalize the downstream flow rate Q with the set flow Q.sub.S.
While the pressure-type flow rate control system has the advantage of controlling the downstream flow rate with precision by detecting the upstream pressure P.sub.1 only, the drawback is that the tiny orifice tends to clog. The orifice is a hole of the order of microns and dust sometimes clogs the orifice hole, rendering the flow rate uncontrollable.
The piping for a fluid to pass through must be well clean. But cuttings, dust, or the like sometimes remain in the piping and clog the orifice. The clogging of the orifice could cause the flow rate control to fail and make the operation of the whole plant unreliable, turning out large quantities of faulty finished products. Furthermore, some gases could become involved in runaway reactions and trigger explosions. Placing a gasket filter in the piping was tried as a solution to the problem, but that did not work because it could have adverse effects on the conductance of the piping.
FIG. 13 shows flow rate characteristics exhibited when the orifice is clogged in comparison with the flow rate characteristics shown by an orifice after it is purged. Flow rate characteristics shown by the orifice after purging constitute normal performance that can be expected of a clean orifice. In FIG. 13, if the set value is 100 percent, for example, the N.sub.2 gas flows at the rate of 563.1 SCCM as indicated by the line with circular marks. The subsequent reaction systems are all designed on the basis of the expected flow rate characteristics of the orifice with no clogging. In the example in FIG. 13, the flow rate given by the clogged orifice decreases to 485 SCCM as indicated by the line with box marks, and the designed reaction can no longer be expected. It is noted that SCCM is the unit of gas flow rate--cm.sup.3 /minute--under the standard conditions.
As shown, clogging of the orifice can make the flow rate lower than the set value. In semiconductor manufacturing and chemical plants, overfeeding or underfeeding of gases as a starting material could trigger an explosion or result in large quantities of faulty finished products. For this reason, how to detect orifice clogging in those gas-using plants has been a major concern.